In many air distribution systems for buildings it is necessary or desirable to provide some type of control over the air moving apparatus so as to maintain a desired pressure, or pressure differential within the system. Systems of this type include, for example, ventilation, air conditioning or heating systems that include one or more fans or blowers and a network of air ducts for distributing air to a plurality of air vents or outlets throughout the building. One or more exhaust intakes are usually provided for returning air to the low pressure side of the blower, and often an intake is provided for introducing a certain amount of fresh or outside air into the system before recirculation through the ductwork to make up for air lost through building exhausts.
In such systems it may be necessary or desirable to monitor the air pressure within the ducts and to provide a control system to operate the fans or blowers to maintain a desired pressure difference between the air pressure in the ducts and the air pressure within the rooms of the building. This type of control may be necessary, for example, to maintain at least a minimum air circulation within the building according to the distribution system design. In other applications pressure control may be needed where the individual heater or ventilator outlets are controlled by damper boxes of the bellows type which are designed to depend upon at least a minimum air pressure differential for proper operation.
Without some type of pressure control, the air distribution system blowers would have to be designed to run continuously at a speed that would ensure proper operation under maximum load conditions with all or most outlets open. This would mean that for most of the time under normal building airflow demand, the blowers would be providing higher than optimum pressure, representing a waste of energy.
In larger systems it is common to use multiple blowers or fans, with a separate blower or blowers for the return air or exhaust from the building and one or more separate blowers to supply the air to the distribution duct system. The outlet of the return blower feeds to the inlet of the supply blower, and in addition a supplemental inlet is usually provided to add a certain amount of outside air to the inlet of the supply blower to make sure that the air circulating within the building stays fresh, and to make up for air escaping from the building through bathroom vents, exhaust hoods, and other air vent paths. Because of these air vent paths, the supply blower generally has to move a larger quantity of air than the return blower, with the difference being the amount of air discharged through the various exhaust vents.
As the various dampers or outlet controls in the air distribution system are modulated to control airflow, for example under local thermostatic control in the case of an air conditioning system, the air flow demand through the distribution ducts and hence the pressure therein will vary accordingly. The object of a pressure control system is to monitor these pressure variations, and to control the blowers so as to vary their output as required to maintain the desired pressure in the system despite variations in the air load requirements of the building. Too low pressure may interfere with proper ventilation or damper operation, and too high pressure will simply waste energy.
As the air flow rate through the system is varied, it is necessary to vary the outputs of both the return and supply blowers. However, the vent loss air flow of the building tends to be relatively constant (excluding large industrial exhaust blowers on an intermittent cycle, discussed in greater detail below) and the required tracking or scheduling of outputs between the return and supply blowers is in general a nonlinear function through the range of air flow for the building. Accordingly, it is necessary for a pressure control system to provide proper adjustment and tracking between the two blowers.
Various pressure control systems for air distribution systems have been proposed in the prior art. One typical widely used system uses pressure transducers or sensors for sensing pressure at a number of points in the distribution duct system, for example adjacent to each damper control box and outlet. These transducer signals are fed to a control box which provides outputs for controlling motors which operate through linkages to control the intake vanes for the blowers so as to control the air volume rate thereof. The outputs from the control box to the actuators are basically proportional to the pressure signals developed by the transducer, with some limited adjustment capability for sensitivity, pressure set point, maximum air volume rate and dead band or pressure range of insensitivity. In systems having both return and supply blowers, the controller provides proportional outputs for both blowers, in some sort of scaled relationship to each other.
Although prior art systems as described above do achieve a useful degree of pressure control, the control they provide is far from optimum, and they are subject to problems in certain areas. One problem with prior art proportional control systems is constant "hunting" or low frequency oscillatory behavior of the system as the pressure varies back and forth across the set pressure. This hunting behavior causes unnecessary wear on the equipment due to the commanding of continual changes in air flow, and can lead to long-term fluctuations in duct pressure which affects the delivery of air in the building. Reducing the sensitivity or gain, or increasing the pressure dead band of the proportional control will reduce hunting, but will lead to slower response, lower accuracy, and greater short-term and long-term errors in the controlled pressure.
Another disadvantage of prior art systems is the imprecise tracking they provide for the supply and return blowers. Specifically, the adjustments or calibrations provided for the high air flow and low air flow settings are interdependent and interrelated, making it impossible or very difficult to calibrate or adjust the system for minimum tracking error at both maximum and minimum air flow delivery rates. Pressure mistracking of the exhaust and supply blowers can be a serious problem. In addition to preventing optimum energy savings, if the tracking error is great enough it could cause excessive positive or negative pressures within the building which could damage the ducts, or conceivably even cause breakage of windows if severe enough. Mistracking leading to a negative pressure within the building also presents the problem of unwanted infiltration of air through small leak paths in the building.